Add pytest coverage for load_config

[dazer1234]

Adds focused pytest coverage for the currently untested load_config helper.

Covered cases:

• reads ~/.config/task-cli/config.yaml from the expected location
• preserves the current missing-config behavior by asserting FileNotFoundError

Validation:

• python -m pytest test_config.py -> 2 passed
• git diff --check

Disclosure: this PR was prepared with assistance from OpenAI Codex and reviewed before submission.